The present invention relates generally to an electric double-layer capacitor, and more particularly to an electric double-layer capacitor having a restorable pressure release valve that is sealed up by a flexible casing material.
Making use of an electric double-layer capacity formed at an interface between a polarizable electrode and an electrolyte, an electric double-layer capacitor has the feature of being much larger in capacity than that comprising electrodes with a dielectric material interposed between them, and is now under development as promising power storage means.
Electric double-layer capacitors known so far in the art are broken down into two types, one type using an aqueous solvent electrolyte and another relying upon a non-aqueous solvent electrolyte. The aqueous solvent type is low in applied voltage due to the electrolysis voltage of water and, in contrast thereto, the non-aqueous solvent type has a great feature of being operated at higher voltage than the aqueous solvent type.
Referring to the electric energy stored in the capacitor, storage efficiency of electric energy increases with an increasing applied voltage, as can be seen from the unit ½ CV2 for electric energy. Thus, the electric double-layer capacitor of the non-aqueous solvent electrolyte type can be well fit for energy storage such as power storage.
Problems with the electric double-layer capacitor of the non-aqueous solvent electrolyte are that as atmospheric moisture enters the electrolyte, the electrolysis voltage of the electrolyte decreases or the non-aqueous solvent electrolyte decomposes. For the electric double-layer capacitor using a non-aqueous solvent electrolyte, therefore, a capacitor enclosure is sealed up to prevent entrance of atmospheric moisture therein.
When the electric double-layer capacitor is used as a condenser, it is of vital importance that the output or energy density of the capacitor is high; the output and energy per volume or weight are large. For this reason, an electric double-layer capacitor enclosed in a flexible casing material obtained by lamination of a synthetic resin film of small mass and an aluminum foil or the like rather than in a metallic can material would be preferable.
The electric double-layer capacitor has an extended charge-discharge cycle life and can be used over an extended period of time, because degradation and deterioration of electrodes are limited due to the absence of such chemical reactions of materials as found in secondary batteries. However, the demerit is that internal pressure rises due to the decomposition of the materials used such as electrolyte and the desorption of moisture adsorbed in the materials during long-term use.
Generally speaking, as internal pressure rises in non-aqueous electrolyte secondary batteries such as lithium ion batteries due to gas generation, abnormal electrochemical reactions proceed to the point where the batteries deteriorate irreparably. For these batteries, therefore, irreparable pressure release valves comprising thin portions that are cleaved or broken are used.
On the other hand, the electric double-layer capacitor has the feature of continuing to work even after internal pressure is released off by operation of the pressure release valve, on condition that the electrolyte is kept in a given amount. For this electric double-layer capacitor, therefore, it is very significant means to rely on a restorable pressure release valve. For such a restorable pressure release valve, a number of arrangements making use of spring resiliency or the like have been generally known in the art. For an electric double-layer capacitor using a flexible casing material such as a synthetic resin film, however, there is still no small-size restorable pressure release valve that is easily joinable to the flexible casing material.
The inventors have already filed Japanese Patent Application No. 2001-80180 to come up with an electric double-layer capacitor comprising a pressure release valve that uses a flexible casing material. Depending on the states of components of the pressure release valve, however, there are inconveniences such as deposition of emissions resulting from the supporting electrolyte materials in the electrolyte onto the outside of the pressure release valve, and a rupture of the flexible casing material.
The inventors have also filed Japanese Patent Application No. 2001-294179 to propose an electric double-layer capacitor to which a pressure release valve having a self-closing passage through it is attached.
This pressure release valve is typically shown in FIG. 13. As shown, a conventional pressure release valve 21 comprises a member of rubber elasticity and a flexible casing material joined directly thereto, and has the feature of being capable of attachment with no additional device.
A problem with this pressure release valve 21 that comprises an elastomer having a self-closing passage and is attached to a flexible casing material 22 is that internal pressure is often not released at a precisely preset pressure due to deformation of the elastomer and the flexible casing material upon an increasing pressure.
Another problem is that upon actuation of the pressure release valve, the pressure release valve does not effectively work due to precipitation of supporting electrolytic materials in the electrolyte.
A primary object of the invention is to provide an electric double-layer capacitor that uses a non-aqueous solvent electrolyte and is sealed up by a flexible casing material, which comprises a pressure release valve that is actuated precisely in response to an abnormal pressure rise to release off internal pressure, so that the electric double-layer capacitor can maintain its own performance over an extended period of time.